Process, plant and overall system for handling and treating a hydrocarbon gas from a petroleum deposit

ABSTRACT

A method of liquefaction/conditioning of a compressed gas/condensate flow extracted from a petroleum deposit, for transport in liquefied form with a transport vessel, especially for such processing of a compressed gas/condensate flow which has been separated from a crude oil extracted from an offshore oil field. The gas/condensate flow is depressurized and cooled in several steps for producing a stabilized liquefied natural gas (LNG) and a stabilized liquefied petroleum gas (LPG), for transport thereof in separate tanks. Disclosed is also a gas expansion plant for execution of the method, and a system for handling and processing of a natural gas from an offshore petroleum field, comprising a production ship to which there is supplied a well stream from an underground source, a field plant installed on the production ship, for processing of the well stream received on the production ship, a vessel for transport of liquefied gas fractions, a high-pressure pipeline arranged for transfer of compressed gas from the field plant to the vessel, and a gas expansion plant according to the invention installed on the transport vessel.

FIELD OF THE INVENTION

The present invention relates to a process and a plant forliquefaction/conditioning of a compressed gas/condensate flow extractedfrom a petroleum deposit, for transport in liquefied form. Morespecifically, the invention relates to a process and a plan t for suchprocessing of a compressed gas/condensate flow which has been separatedfrom a crude oil extracted from an offshore oil field, for transportthereof in liquefied form with a transport vessel. The invention alsorelates to an overall system for handling and processing of natural gasfrom an offshore petroleum field, for transport of the gas in liquefiedform with a vessel f or transport of liquefied gas fractions.

BACKGROUND OF THE INVENTION

In the production of crude oil from an offshore oil field there iscarried out a separation of the well stream into water, oil and naturalgas. The natural gas following the produced crude oil in the wellstream, and which is commonly designated "associated gas", will often bedesired to be transported from the offshore field to a receiving systemon land.

An obvious thought has been to build a plant for the production ofliquefied associated gas at a production platform or a production shipwhich is installed on the field, and which receives the well stream forprocessing. After separation of the stream into water, oil and gas, theseparated gas should be able to be condensed to a liquid condition inwhich it has a low pressure and a low temperature, and thereafter to betransferred through a pipeline system to a vessel for transport to landin this condition. However, this is not feasible in a practical mannerwith the technique of today, since cryogenic transfer of liquefiednatural gas via conventional "loading arms", or even via moresophisticated transfer systems, is associated with hitherto unsolvedproblems with freezing, clogging of passages, etc. Such transfer is alsoassociated with danger of an unintended spill of liquefied natural gasonto the sea, which might result in explosion-like evaporation, with asubstantial destructing potential.

I order to avoid the cryogenic transfer of condensed gas from theproduction platform or the production ship to the transport vessel, onecould instead think of transferring the natural gas from the productionplatform or ship to a transport vessel equipped with a completeconventional plant for liquefaction of the natural gas, for productionof mainly LNG. However, conventional plants for the production of LNGare very expensive, and it is therefore not economically acceptable tobuild such plants on individual transport vessels.

Another proposal for solving the problem is described in U.S. Pat. No.5,025,860. Here is described a system wherein a natural gas from anoffshore petroleum deposit is carried to a production platform orproduction ship where carbon dioxide and water are separated from thenatural gas, whereafter the natural gas is compressed and cooled to acompressed gas condition. In this gas condition, under a high pressure,the natural gas is thereafter transported through a pipeline system toan LNG tanker where it is expanded and cooled to form of liquefiednatural gas (LNG) which is stored on board in the tanks of the ship. Inthe expansion and cooling process on board the LNG tanker there is alsoobtained a non-condensed residual gas which can be carried back to theproduction platform or ship.

Advantages which are achieved with this natural gas processing systemaccording to U.S. Pat. No. 5,025,860 are stated to be that practicallyall of the energy required for liquefying the gas on board theLNG-tanker is supplied to the gas on board the production platform orship. Consequently, the investment cost of the necessary and expensiveinstallation for supply of this energy to the gas will be connected tothe production platform or ship, whereas every single one of theLNG-tankers which is to transport the natural gas, only needs arelatively limited production equipment on board, which in addition canbe mounted on the upper deck of the ship, on replaceable framestructures suitable for-the purpose.

The natural gas which is of current interest to be conditioned fortransport by means of this system according to U.S. Pat. No. 5 025 860,may come from a natural gas source, or it may be a by-product from anoil source (associated gas).

That part of the natural gas processing system according to U.S. Pat.No. 5,025,860 which is installed on board the production platform orship, and which has for its purpose to purify the natural gas andthereafter to compress and cool the gas for delivery to the LNG-tankerin compressed gas condition, is designed as a traditional plant for thispurpose.

That part of the natural gas processing system according to U.S. Pat.No. 5,025,860 which is installed on board the LNG tanker, comprises anexpansion plant, wherein the received, cooled high-pressure gas issubjected to an additional cooling and is expanded adiabatically inthree stages. A liquefied LNG gas with a pressure of ca. 1 bar istransported as a final product from the expansion plant to storage tankson board, prepared for transport. Non-condensed gas from the expansionplant is carried through a compression group where it is compressed to apressure of ca. 30 bar, whereafter it is returned to the productionplatform or ship through a return line, for use e.g. as a fuel foroperation for the compressors for compression and cooling of the naturalgas on board the production platform or ship.

SUMMARY OF THE INVENTION

It has now been found that a natural gas processing system of thedescribed type, wherein the natural gas is subjected to an introductorypurification and is compressed to a desired high pressure on aproduction platform or production ship and thereafter is transferred inthis compressed condition to a transport vessel to be expanded andliquefied there for transport in liquefied form, can be substantiallyimproved. This is achieved in that the compressed natural gas, which mayexist as a gas, as a two-phase mixture of gas and liquid or as aso-called "dense phase", and which is here designated as agas/condensate flow, is subjected to expansion on board the transportvessel in a specific manner which entails that the entire gas/condensateflow can be stored in stable condition, cooled and at approximatelyatmospheric pressure, as two distinct products for separate transportwith the transport vessel, viz. as LNG and a heavier, liquid petroleumgas LPG (Liquefied Petroleum Gas). This solution gives a goodflexibility for the processing of a wide range of gas/condensatequalities.

Thus, with the invention there is provided a method ofliquefaction/conditioning of a compressed gas/condensate flow extractedfrom a petroleum deposit, for transport in liquefied form, especiallyfor such processing of a gas/condensate flow which has been separatedfrom a crude oil extracted from an offshore oil field, for transportthereof in liquefied form with a vessel for transport of liquefied gasfractions.

With the invention there is moreover provided a plant for execution ofthe novel method.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described below in connection withembodiments with reference to the annexed drawings, wherein

FIG. 1 shows a plant according to the invention for expansion andcondensation of an associated gas under a high pressure from an offshoreproduction platform or a production ship; and

FIG. 2 is a schematic view showing an overall system for processing ofan associated gas from an offshore petroleum field, for transport of thegas in liquefied form with a vessel for transport of liquefied gasfractions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1 there is firstly described an embodiment of themethod according to the invention in a plant according to the inventioninstalled on board a transport vessel.

A flow 1 of gas and condensate, which has been subjected to drying andremoval for CO₂ in a common known manner, and which, with a pressure of20-500 bar, especially 100-350 bar, and a temperature in the range from4° C. to 50° C., is supplied through a pipeline from a productionplatform or a production ship, is carried via one or more conventionaldriers 2 to a first pressure relief valve 3, a so-called Joule-Thomsonvalve. Possibly there may be used several such valves. As an alternativeto such an expansion valve, there may be used an isentropic expansionturbine (turbo expander).

After an adiabatic depressurization in the expansion valve 3 to apressure in the range 40-70 bar and a temperature in the range from +10°C. to -60° C., the flow is introduced to a phase separator 4 wherein itis separated into a gas phase and a liquid phase. The gas phase from thephase separator 4 is carried via a unit 5 for removal of mercury to apipe coil heat exchanger 10 wherein it is cooled. To remove mercury fromthis gas phase is necessary for preventing corrosion of the structuralmaterial in the heat exchanger.

From the heat exchanger 10 the cooled flow is carried to a secondpressure relief valve 6. The pressure after the adiabaticdepressurization undertaken in the valve 6 may be ca. 5 bar lower thanthe pressure after the first pressure relief valve 3. As a result ofthis depressurization there takes place a condensation of heavierhydrocarbons including aromatics. These components have to be removedbecause, similar to water and CO₂, they will be able to freeze out andclog the process equipment if they are not removed to a sufficiently lowlevel. The depressurized flow from the valve 6 is introduced into aphase separator 7, wherein it is separated into a gas phase and a liquidphase containing said heavier hydrocarbons. The gas phase from the phaseseparator 7 is carried to a phase separator 8 connected in series withthis separator and from which a liquid phase is returned to the phaseseparator 7. The gas phase 8a from the phase separator 8 is carried to aunit 9 wherein CO₂ is removed to a level preventing freezing-out withfurther cooling of the flow, and therefrom to the above-mentioned pipecoil heat exchanger (10), where in the gas phase is condensed andsupercooled.

The flow of condensed and supercooled gas from the heat exchanger 10,which has now a pressure close to the atmospheric pressure, thereafteris further pressurized in a third pressure relief valve 11, and theoutlet flow therefrom is introduced into a phase separator 12 wherein itarrives with a temperature of from -158° C. to -163° C. After a possibleadditional let-down of the pressure to a pressure just above theatmospheric pressure, the liquid phase 12b from this phase separator iscarried to storage in storage tanks 13 at approximately -163° C., as astabilized liquefied natural gas (LNG). From the phase separator 12there is also taken out a gas phase 12a consisting of a lighthydrocarbon gas enriched with nitrogen. This gas may be utilized as afuel for power-demanding machinery in the plant or in an associatedplant (not shown) for example on board the production platform or ship.

The flow 1 of gas and condensate which is supplied to the plant,preferably has such a pressure that the depressurization in the firstpressure relief valve 3 can be undertaken to a pressure in the range60-70 bar.

The liquid phase which is separated in the phase separator 4 after thefirst depressurization of the gas/condensate flow 1 in the valve 3, iscarried to a fourth pressure relief valve 14. The depressurized flowtherefrom, which has now an overpressure of 1-2 bar and a temperature offrom -30 to -55° C., is mixed in a mixing device 15 with the liquidphase 7a from the phase separator 7, and the mixed flow is carried to aheat exchanger 16 wherein, if necessary, there is undertaken anadjustment of the temperature of the flow. From the heat exchanger theflow is carried to a phase separator 17 from which a liquid phaseconsisting of stabilized liquefied production gas (LPG) is carried tostorage tanks 18. This liquefied production gas mainly consists of amixture of propane and butanes, but it may also contain substantialamounts of methane and components which are heavier than butane.

I the heat exchanger 10 there is used a cryogenic cooling medium from acooling plant 19 comprising a driving unit 20 and a compressor 21. Thecryogenic cooling medium circulates in a closed cooling circuit and forexample may be constituted by nitrogen-containing hydrocarbon gasseparated in the phase separator 12.

The plant shown in FIG. 1 preferably is driven without any recirculationof non-condensed hydrocarbon flows from the phase separators 12 and 17,and preferably there is used only one driving unit 20. As driving unit20 it is preferred to use a gas turbine.

For the removal of CO₂ in the unit 9 there may be used a traditionalmolecular sieve equipment, which is very robust against movements (heavysea). If desired, separated CO₂ can be recompressed and returned to thereservoir.

By means of the method and the plant shown in FIG. 1 there is achievedthat all the associated gas which is separated in a processing plant onboard a production ship or a production platform on an offshore oilfield, and which is supplied to the plant according to the invention incompressed form, is able to be handled. The gas flow is condensed into aheavier portion (LPG) and a light portion (LNG), which are stored instable form individually, cooled down and at approximately atmosphericpressure on board the transport vessel. The method and plant are notenergy optimal, but give great savings on the investment side, and theyare flexible with respect to enabling the handling of a wide range ofgas qualities. In addition, the plant is robust to heavy sea and is inits entirety able to be installed on a single module on board thetransport vessel.

The method and plant according to the invention described above may, asappears from the above, advantageously form part of an overall systemfor processing of a gas/condensate flow from an offshore oil or gasfield for transport in liquefied form with a transport vessel.

In offshore production of hydrocarbons (oil and gas) it is known to useproduction ships which are based on the so-called STP technique(STP=Submerged Turret Production). In this technique there is used asubmerged buoy of the type comprising a central, bottom-anchored membercommunicating with the topical underground source via at least oneflexible riser, and which is provided with a swivel unit for thetransfer of the fluid under a high pressure to a production plant on theship. On the central buoy member there is rotatably mounted an outerbuoy member which is arranged for introduction and releasable securingin a submerged downwardly open receiving space at the bottom of theship, so that the ship can turn about the anchored, central buoy memberunder the influence of wind, waves and water currents. For a furtherdescription of this technique there may e.g. be referred to Norwegianlaying-open print No. 175 419.

In offshore loading and unloading of hydrocarbons it is further known touse a so-called STL buoy (STL=Submerged Turret Loading) which is basedon the same principle as the STP buoy, but which has a simpler swiveldevice than the STP swivel which normally has several through-goingpassages or courses. For a further description of this buoy structurethere may e.g. be referred to Norwegian laying-open print No. 176 129.

By means of the STP/STL technique there is achieved that one can carryout loading/unloading as well as offshore production of hydrocarbons innearly all weathers, both connection and disconnection between ship andbuoy being able to be carried out in a simple and quick manner, alsounder very difficult weather conditions with high waves. Further, thebuoy can remain connected to the ship in all weathers, a quickdisconnection being able to be carried out if a weather limitationshould be exceeded.

Because of the substantial practical advantages involved in the STP/STLtechnique, it would be desirable to be able to use this technique alsoin connection with the utilization of the natural gas (associated gas)produced together with the oil in offshore oil production.

Thus, with the invention there is also provided an overall system forhandling and processing of a natural gas from an offshore petroleumfield, for transport of the gas in liquefied form with a transportvessel. That which is characteristic of the system according toinvention, is stated in the characterizing part of claim 19. Variousembodiments of the system are stated in the dependent claims 20-23.

The fundamental construction of the new total system according to theinvention for processing of a compressed gas/condensate flow from anoffshore oil or gas field for transport in liquefied form with atransport vessel is schematically shown in FIG. 2.

In the illustrated embodiment the system comprises a floating productionship 31 on which there is provided a field plant or installation 32 forprocessing of a well stream flowing up from an underground source 33.The well stream is supplied via a wellhead 34 and a flexible riser 35which extends through the body of water 36 and at its upper end isconnected to an STP buoy 37 of the above-mentioned type. The buoy isintroduced and releasably secured in a submerged downwardly openreceiving space 38 at the bottom of the production ship 31. As mentionedabove, the buoy comprises a swivel unit forming a flow connectionbetween the riser 35 and a pipe system (not shown) provided on theproduction ship between the swivel and the field installation 32. Thecentral member of the buoy is anchored to the sea bed 39 by means of asuitable anchoring system comprising a number of anchor lines 40 (onlypartly shown). For a further description of the buoy and swivelconstruction reference is made to the aforementioned Norwegianlaying-open print 176 129.

The field installation 32 consists of a number of processing units ormodules 41 for suitable processing of the supplied well stream from thesource 33. After separation of the well stream into water, oil and gas,the gas, which is that part of the well stream which is here ofinterest, is subjected to drying and removal of CO₂ in a usual knownmanner.

After this treatment of the gas, the gas is compressed to a desired highpressure of at least 150 bar, whereby--as a result of the compression--aheating of the gas to a relatively high temperature takes place. The gasnow exists in a condition which is optimal with a view to expansion ofthe gas to liquid form in an expansion plant according to the invention,which will be substantially more reasonable to build than a conventionalLNG plant. However, in certain cases it may be advantageous to cool thecompressed gas "maximally" before the gas is supplied to the expansionplant, which is located on board the transport vessel 45.

A flexible pipeline 44 which is arranged for transfer of the compressedgas, extends through the body of water (the sea water) 36 between theproduction ship 31 and the transport vessel 35. One end of the pipelineat the production ship 31 is permanently connected to the STP buoy 37and is connected to the field installation 32 via the swivel unit of thebuoy and said pipe system on the production ship. The other end of thepipeline 44 is permanently connected to an additional STP buoy 46 whichis introduced and releasably secured in a submerged downwardly openreceiving space 47 in the transport vessel 45. The buoy is provided witha swivel unit which may be of a similar design as that of the swivelunit in the buoy 37, and its central member is anchored to the sea bed39 by means of an anchoring system comprising a number of anchor lines48.

In addition to the buoy 46 (buoy I) there is also provided an additionalsubmerged buoy 49 (buoy II) which is anchored to the sea bed by means ofanchor lines 50. The pipeline 44 is also permanently connected to thisbuoy via a branch pipeline in the form of a flexible riser 44' which isconnected to the pipeline 44 at a branch point 51. The purpose of thearrangement of two buoys will be further described later.

The pipeline 44 may extend over a substantial length in the sea, asuitable distance between the production ship 31 and the buoys I and IIin practice being 1-2 km. When compressed gas with a high temperature isto be transferred from the field installation 32 through the pipeline,this has been made heat transferring, so that the gas temperature duringthe transfer is lowered to a desired low temperature close to the seawater temperature, e.g. 4-10° C. On the other hand, when compressed gasat a low temperature is to be transferred, the pipeline has been madeheat-insulating, so that the gas temperature is maintained during thetransfer.

A plant 52 according to the invention, for expansion and cooling ofcompressed gas to liquid form, is installed on board the transportvessel 45. The plant is supplied with compressed gas from the pipeline44, which communicates with the plant via the buoy 46 and a pipe system(not shown) on the transport vessel 45. Liquefied LNG and LPG which areproduced in the plant, are stored in tanks 53 on board the transportvessel.

It will often be of interest to transfer residual gas from the expansionplant 52 back to the production ship 31 for recompression/cooling. Insuch cases the pipeline 44 may also comprise a return line for transferof such gas from the expansion plant back to the production ship. Insome cases it will further be expedient to produce electrical energy asa byproduct from the expansion process in the plant 52. In such casesthe pipeline 44 may also comprise a power cable for transfer of electriccurrent from the transport vessel 45 to the production ship 31, as theswivel units of the STP buoys may be constructed for such transfer.

As shown in FIG. 2, the transport vessel 45 is coupled to the loadingbuoy 46 (buoy I), whereas the additional buoy 49 (buoy II) is submerged,waiting for connection to another transport vessel. In practice one canenvisage that the expansion plant 52 can produce e.g. ca. 8 000 tons LNGper day. With a ship size of 80 000 tons the transport vessel 45 thenwill be able to lie connected to the buoy I for about 10 days before itsstorage tanks 53 are full. When the tanks are full, the vessel leavesbuoy I, and the production continues via buoy II where another transportvessel then is connected. The ready-loaded vessel transports its cargoto a receiving terminal. Based on normal transport distances and saidloading time, for example four transport vessels may be connected to theshown arrangement with two buoys I and II, thereby to obtain operationwith "direct shuttle loading" (DSL) without any interruption in theproduction.

Even if direct shuttle loading may be achieved with the shownarrangement, a continuous take-off of gas is not always an absolutepresupposition, so that a transport vessel does not need to becontinuously coupled to one of the loading buoys. Thus, the transportvessel may leave the field/buoy for at least shorter time periods (somedays) without this having negative consequences.

What is claimed is:
 1. A method of liquefaction/conditioning of acompressed gas/condensate flow (1) extracted from a petroleum deposit,for transport in liquefied form, especially for such processing of acompressed gas/condensate flow which has been separated from a crude oilextracted from an offshore oil field for transport thereof in liquefiedform with a vessel for transport of liquefied gas fractions, wherein(a)the gas/condensate flow (1) is depressurized (3) in a firstdepressurizing step to a pressure in the range 40-70 bar and atemperature in the range from +10° C. to -60° C. and thereafter isseparated into a gas phase and a liquid phase in a phase separator (4),(b) the gas phase from the phase separator (4) is cooled in a heatexchanger (10), (c) the cooled gas phase from the heat exchanger (10) isdepressurized adiabatically (6) in a second depressurizing step, withsubsequent separation into a gas phase (8a) and a liquid phase (7a) inone or more serially connected phase separators (7, 8), (d) the gasphase (8a) from the second depressurizing step is carried to the heatexchanger (10) where it is condensed and supercooled, (e) the liquidphase from the heat exchanger (10) is depressurized (11) in a thirddepressurizing step and carried at a temperature of from -158 to -163°C. to a final phase separator (12) wherein a light nitrogen-enrichedhydrocarbon gas (12a) is separated from a liquid phase (12b), thepressure of the extracted liquid phase (12b) is let down, and thisliquid phase, consisting of a stabilized liquefied natural gas (LNG), iscarried to be stored in storage tanks (13) at approximately -163° C. anda pressure at or just above the atmospheric pressure, and (f) the liquidphases from the phase separators (4 resp. 7) associated with the firstand second depressurizing steps (3 resp. 6) are converted bydepressurization, temperature control (16), and final phase separation(17) to a liquid phase consisting of a stabilized liquefied petroleumgas (LPG) and a gas phase.
 2. A method according to claim 1, wherein thedepressurization in each of the four depressurizing steps is carried outadiabatically, through one or more Joule-Thomson valves (resp. 3, 6, 11and 14).
 3. A method according to claim 1 or 2, wherein thedepressurization (3) in the first depressurizing step (step a) iscarried out at a pressure in the range 60-70 bar.
 4. A method accordingto claim 1, wherein, as a heat exchanger (10), there is used a pipe coilheat exchanger.
 5. A method according to claim 1, wherein, in the seconddepressurizing step (step c), there are used two series-connected phaseseparators (7, 8).
 6. A method according to claim 1, wherein thedepressurization (6) in the second depressurizing step (step c) iscarried out at a pressure which is approximately 5 bar lower than thepressure after the first depressurizing step (step a).
 7. A methodaccording to claim 1, wherein the following two steps (f) and (g):(f)the liquid phase from the phase separator (4) of the firstdepressurizing step is depressurized (14) in a fourth depressurizingstep to an overpressure of 1-2 bar and a temperature from -30 to -55° C.and thereafter is mixed in a mixing device (15) with the liquid phase(7a) from the second depressurizing step, and (g) the mixed liquid phasefrom the mixing device (15), after an adjustment of the temperature in aheat exchanger (16), is separated in a final phase separator (17) fromwhich a liquid phase consisting of stabilized liquefied petroleum gas(LPG) is carried to storage tanks (18).
 8. A method according to claim1, wherein, as a cooling medium in the heat exchanger (10), there isused a cryogenic cooling medium which circulates in a closed coolingcircuit and is cooled and condensed in a cooling plant (19) comprising adriving unit (20) and a compressor (21).
 9. A method according to claim8, wherein the driving unit (20) is a gas turbine.
 10. A methodaccording to claim 1, wherein, as a cryogenic cooling medium in the heatexchanger (10), there is used a nitrogen-containing, light hydrocarbongas separated in the further phase separator (12).
 11. A methodaccording to claim 1, wherein that nitrogen-containing, lighthydrocarbon gas separated in the further phase separator (12) and/or gasseparated in a final phase separator (17), is used as a fuel forpower-demanding machinery in the plant or an associated plant.
 12. Amethod according to claim 1, wherein it is carried out withoutrecirculation of non-condensed hydrocarbon flows and by the use of onlyone driving unit (20).
 13. A plant for liquefaction/conditioning of acompressed gas/condensate flow (1) extracted from a petroleum deposit,for transport in liquefied form, especially for such processing of acompressed gas/condensate flow which has been separated from a crude oilextracted from an offshore oil field for transport thereof in liquefiedform with a vessel for transport of liquefied gas fractions,comprising:(a) a first pressure relief device (3) for depressurizing thegas/condensate flow (1) to a pressure in the range 40-70 bar and atemperature in the range from +10° C. to -60° C., and a first phaseseparator (4) for separation of the flow from the pressure relief device(3) into a gas phase and a liquid phase, (b) a second pressure reliefdevice (6) for adiabatic depressurization of the gas phase from thefirst phase separator (4) after previous cooling of this gas phase, andone or more series-connected phase separators (7, 8) for separation ofthe flow from the second pressure relief device (6) into a gas phase(8a) and a liquid phase (7a), (c) a heat exchanger (10) for cooling ofthe gas phase from the first phase separator (4) and for condensing andsupercooling the gas phase (8a) from the series-connected phaseseparator(s) (7, 8), (d) a third pressure relief device (11) foradiabatic depressurization of the gas phase condensed and supercooled inthe heat exchanger (10) and coming from the series-connected phaseseparator(s) (7, 8), and a further phase separator (12) for separationof the flow from the pressure relief device (11) into a gas phase (12a)and a liquid phase (12b) consisting of stabilized liquefied natural gas(LNG), (e) storage tanks (13) for reception and storage of the liquidphase (12b) consisting of stabilized liquefied natural gas (LNG), (f) afourth pressure relief device (14) for adiabatic depressurization of theliquid phase from the phase separator (4) to an overpressure in therange 1-2 bar and a temperature in the range of -30° C. to -55° C., (g)devices for depressurizing (14), temperature control (16) and phaseseparation (17) of the liquid phases from the liquid phase from thephase separators (4, 7) associated with first and fourth pressure reliefdevices (3, 6) for achieving a stabilized liquefied petroleum gas (LPG)and a gas phase, (h) storage tanks (18) for reception and storage of theliquid phase consisting of the stabilized liquefied petroleum gas (LPG),and (i) a cooling plant (19) for delivery of a cooling medium to theheat exchanger (10) in a closed cooling circuit, which cooling plantcomprises a driving unit (20) and a compressor (21).
 14. A plantaccording to claim 13, wherein each of the pressure relief devices (3,6, 11, 14) is constituted by one or more Joule-Thomson valves.
 15. Aplant according to claim 13, wherein the heat exchanger (10) is a pipecoil heat exchanger.
 16. A plant according to claim 13, furthercomprising two phase separators (7, 8) for the second pressure-reliefstep.
 17. A plant according to claim 13, further comprising:a mixingdevice (15) for mixing of the depressurized flow from the fourthpressure-relief device (14) with the liquid phase (7a) from theseries-connected phase separator(s) (7, 8), and a further heat exchanger(16) for adjusting the temperature of the mixture flow from the mixingdevice (15), and a phase separator (17) for separation of the flow fromthe further heat exchanger (16) into a gas phase and a liquid phaseconsisting of stabilized liquefied petroleum gas (LPG).
 18. A plantaccording to claim 13, wherein the driving unit (20) of the coolingplant (19) is a gas turbine.
 19. A system for handling and processing ofa natural gas from an offshore petroleum field, for transport of the gasin liquefied form with a transport vessel, comprising:(A) a productionship (31) to which there is supplied a well stream from an undergroundsource (33), (B) a field installation (32) installed on the productionship (31), for processing of the well stream received on the productionship, including separation of the well stream into water, oil, and gas,which field installation comprises a sub-installation for purifying gasseparated from the well stream and for compressing and cooling this gasto a desired high pressure and a desired temperature, (C) a vessel (45)for transport of liquefied gas fractions, (D) a high-pressure pipeline(44) which is arranged for transfer of the compressed gas from the fieldinstallation (32) to the vessel (45), and which extends through asurrounding body of water (36), which pipeline (44) at the end which isconnected to the field installation (32), is permanently coupled to aloading buoy (37) arranged for introduction and releasable securing in asubmerged downwardly open receiving space (38) at the bottom of theproduction ship (31), and which is provided with a swivel unit fortransfer of gas under a high pressure, the swivel unit also beingcoupled to a transfer line (35) communicating with the undergroundsource (33), and at the end which is remote from the field installation(32) is permanently coupled to at least one loading buoy (46) arrangedfor introduction and releasable securing in a submerged downwardly openreceiving space (47) at the bottom of the vessel (45), and which isprovided with a swivel unit for transfer of gas under a high pressure,and (E) a gas expansion plant (52) according to claim 13 installed onthe transport vessel (45).
 20. A system according to claim 19, whereinthe pipeline (44) is coupled to two loading buoys (46, 49) viarespective flexible risers.
 21. A system according to claim 19, whereinthe loading buoys (37, 46, 49) are STP buoys.
 22. A system according toclaim 19, wherein the pipeline (44) also comprises a return line fortransfer of residual gas from the expansion plant (52) back to the fieldplant (32).
 23. A system according to claim 19, wherein the pipeline(44) also comprises a power line for transfer of electric current to thefield plant (32) from a power-producing device driven by surplus energygenerated by operation of the expansion plant (52).